Permanent magnet motors offer significant potential benefits for applications in which an electric motor is utilized to drive a refrigerant compressor including enhanced efficiency, power density, and speed control precision. Such motors also present controls and diagnostic challenges. The temperature of the magnetic material of such motors must be controlled to avoid damage from elevated temperature conditions which can arise, for example, from inadequate cooling or increased stator or rotor loss. Furthermore, some applications present a risk of chemical or mechanical attack on the magnets. Conventional techniques for controlling and diagnosing permanent magnet motors suffer from a number of shortcomings including imprecision, computational complexity and inefficiency, frequency dependence, response speed, and microprocessor-burdening division operations, among other shortcomings. There is a need for the unique and inventive controls and diagnostics disclosed herein.